[PATCH v2 1/2] virtio_ring: reject a split ring size that is not a power of two again

[Stjepan Poljak <stjepan.poljak@gmail.com>]

The split ring indexes its avail and used rings with num - 1 as a mask,
so its size must be a power of two. Commit 2a2d1382fe9d ("virtio: Add
improved queue allocation API") moved that check from
vring_new_virtqueue() into the new allocation path, which
vring_new_virtqueue() does not use.

remoteproc calls vring_new_virtqueue() with a ring size from the
firmware, checked only to be non-zero. With 100 entries, for example,
the driver would put buffers 4 and 8 in avail slot 0. The device would
look for them at slots 4 and 8, possibly reading the wrong data.

Put the check back for the split ring; the packed ring has no such
restriction.

Fixes: 2a2d1382fe9d ("virtio: Add improved queue allocation API")
Signed-off-by: Stjepan Poljak <stjepan.poljak@gmail.com>
---
remoteproc still reports this as -ENOMEM, although the "Bad virtqueue
length" error names the cause. Returning an error pointer from
vring_new_virtqueue() is left for later; the same change for
vring_create_virtqueue() is here:
https://lore.kernel.org/r/20261106055635.3019889-1-stjepan.poljak@gmail.com

v2:
- Log with dev_err() rather than dev_warn(), and say in the message why
  the length is bad (Francesco)
---
 drivers/virtio/virtio_ring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index f683a2d6a200..f855b8d41575 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -3513,6 +3513,13 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 						    name, map);
 	}
 
+	/* The split ring indexes its rings with num - 1 as a mask. */
+	if (!is_power_of_2(num)) {
+		dev_err(&vdev->dev,
+			"Bad virtqueue length %u (not a power of two)\n", num);
+		return NULL;
+	}
+
 	vring_init(&vring_split.vring, num, pages, vring_align);
 	return __vring_new_virtqueue_split(index, &vring_split, vdev, weak_barriers,
 				     context, notify, callback, name,
-- 
2.53.0